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Increasing competitive pressure drives the automotive industry to optimise the manufacturing processes in order to reduce the production costs. Within the framework of this ECSC project steel manufacturers, a steel user and research institutes aimed to develop a new generation of fine grain case-hardening steels, allowing an increase of carburising temperature resulting in an abbreviate carburising time. Because of the numerous effects of the austenite grain size on the mechanical properties, an improper austenite grain growth has to be avoided. The target was to control grain size with a distribution of 90 vol.% > ASTM 5 for a heat treatment of 1050°C/5 hours.

The investigations showed that austenite grain growth can be prevented, following two different routes:
a) A combination of 0.025 mass% Al, 0.035 mass% Nb, 0.010 mass% Ti together with a balanced content of nitrogen around 120 ppm, thus leading to a homogenous precipitation of small nitrides and carbonitrides.
b) A higher content of nitrogen (over 200 ppm) blocked only with Al (0.035 mass%) and b (0.04 mass%) thus provoking higher temperature (and therefore more stable) fine precipitates.

It has to be considered that not only the chemical composition but also the parameters of the complete manufacturing process has to be matched to the formation of the grain boundary pinning precipitations. There must be a significant amount of precipitates < 30 nm in the material to prevent grain growth.

Additional information

Authors: LEPPANEN R, Ovako Steel AB, Hallefors (SE);BLECK W, IEHK-RWTH, Aachen (DE);HIPPENSTIEL F, IEHK-RWTH, Aachen (DE);HOFFMANN F, Institut für Werkstofftechnik, Bremen (DE);PICHARD C, Ascométal, Hagondange (FR);KUEPER A, ZF Friedrichshafen AG, Friedrichshafen (DE);SMITH H, Corus UK, Middlesbrough (GB);LEPPÄNEN R, Ovako Steel AB, Hällefors (SE);BERTRAND C, Sidenor I+D, Basauri (ES)
Bibliographic Reference: EUR 20630 EN (2003), pp.209. Euro: 35.00
Availability: EUR-OP reference: KI-NA-20630-EN-S Available from EUR-OP sales agents URL:
ISBN: ISBN: 92-894-5195-5
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